Magnetically enhanced inductively coupled plasma reactor with magnetically confined plasma

ABSTRACT

The invention is embodied in an RF plasma reactor for processing a semiconductor wafer, including as reactor chamber bounded by a chamber wall defining an interior region of the chamber, a gas inlet, an RF power source and an RF power applicator proximal the chamber and connected to the RF power source, and an opening in this chamber communicating with the interior region of the chamber. The invention further includes a magnet apparatus disposed adjacent said opening to resist flow of plasma ions through the opening, and the magnet apparatus comprising a first pair of magnetic poles and a second pair of magnetic poles, the first pair of magnetic poles facing the second pair of magnetic poles across the opening.

This is a continuation of U.S. application Ser. No. 09/263,001, filed Mar. 5, 1999, which is a continuation-in-part of U.S. application Ser. No. 08/766,119, filed Dec. 16, 1996, now U.S. Pat. No. 6,036,486 which is a continuation of now-abandoned U.S. application Ser. No. 08/590,998, filed Jan. 24, 1996 now abandoned.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention is related to plasma reactors for processing semiconductor wafers, and in particular confinement of the processing plasma in the reactor within a limited processing zone.

2. Background Art

Plasma reactors, particularly radio frequency (RF) plasma reactors of the type employed in semiconductor wafer plasma processing in the manufacturing of microelectronic integrated circuits, confine a plasma over a semiconductor wafer in the processing chamber by walls defining a processing chamber. Such an approach for plasma confinement has several inherent problems where employed in plasma reactors for processing semiconductor wafers.

First, the walls confining the plasma are subject to attack from ions in the plasma, typically, for example, by ion bombardment. Such attack can consume the material in the walls or introduce incompatible material from the chamber walls into the plasma process carried out on the wafer, thereby contaminating the process. Such incompatible material may be either the material of the chamber wall itself or may be material (e.g., polymer) previously deposited on the chamber walls during plasma processing, which can flake off or be sputtered off. As one example, if the chamber walls are aluminum and the plasma process to be performed is plasma etching of silicon dioxide, then the material of the chamber wall itself, if sputtered into the plasma, is incompatible with the process and can destroy the integrity of the process.

Second, it is necessary to provide certain openings in the chamber walls and, unfortunately, plasma tends to leak or flow from the chamber through these openings. Such leakage can reduce plasma density near the openings, thereby upsetting the plasma process carried out on the wafer. Also, such leakage can permit the plasma to attack surfaces outside of the chamber interior. As one example of an opening through which plasma can leak from the chamber, a wafer slit valve is conventionally provided in the chamber side wall for inserting the wafer into the chamber and withdrawing the wafer from the chamber. The slit valve must be unobstructed to permit efficient wafer ingress and egress. As another example, a pumping annulus is typically provided, the pumping annulus being an annular volume below the wafer pedestal coupled to a vacuum pump for maintaining a desired chamber pressure. The chamber is coupled to the pumping annulus through a gap between the wafer pedestal periphery and the chamber side wall. The flow of plasma into the pumping annulus permits the plasma to attack the interior surfaces or walls of the pumping annulus. This flow must be unobstructed in order for the vacuum pump to efficiently control the chamber pressure, and therefore the pedestal-to-side wall gap must be free of obstructions.

It is an object of the invention to confine the plasma within the chamber without relying entirely on the chamber walls and in fact to confine the plasma in areas where the chamber walls to not confine the plasma. It is a related object of the invention to prevent plasma from leaking or flowing through openings necessarily provided the chamber walls. It is an auxiliary object to so prevent such plasma leakage without perturbing the plasma processing of the semiconductor wafer.

It is a general object of the invention to shield selected surfaces of the reactor chamber interior from the plasma.

It is a specific object of one embodiment of the invention to shield the interior surface of the reactor pumping annulus from the plasma by preventing plasma from flowing through the gap between the wafer pedestal and the chamber side wall without obstructing free flow of charge-neutral gas through the gap.

It is a specific object of another embodiment of the invention to prevent plasma from flowing through the wafer slit valve in the chamber side wall without obstructing the ingress and egress of the wafer through the wafer slit valve.

SUMMARY OF THE DISCLOSURE

The invention is embodied in an RF plasma reactor for processing a semiconductor wafer, including as reactor chamber bounded by a chamber wall defining an interior region of the chamber, a gas inlet, an RF power source and an RF power applicator proximal the chamber and connected to the RF power source, and an opening in this chamber communicating with the interior region of the chamber. The invention further includes a magnet apparatus disposed adjacent said opening to resist flow of plasma ions through the opening, and the magnet apparatus comprising a first pair of magnetic poles and a second pair of magnetic poles, the first pair of magnetic poles facing the second pair of magnetic poles across the opening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut-away side view of a plasma reactor in accordance with a first embodiment of the invention employing open magnetic circuits.

FIG. 2 is an enlarged view of the magnetic confinement apparatus near the pedestal-to-side wall gap.

FIG. 3 is an enlarged view of the magnetic confinement apparatus near the wafer slit valve.

FIGS. 4A and 4B correspond to a side view of a plasma reactor in accordance with a preferred embodiment of the invention employing closed magnetic circuits having pairs of opposed magnets.

FIG. 5 is a perspective view of a pair of opposing ring magnets juxtaposed across the pedestal-to-side wall gap.

FIG. 6 is a perspective view of a pair of opposing magnets juxtaposed across the wafer slit valve.

FIG. 7 is a cut-away side view of a plasma reactor in which the closed magnetic circuit is a single magnet whose opposing poles are juxtaposed across the pedestal-to-side wall gap and which are joined by a core extending across the pumping annulus.

FIG. 8 is a top view of the single magnet of FIG. 7 and showing the gas flow holes through the core joining the opposite poles of the magnet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Conventional Reactor Elements

Referring to FIG. 1, an RF plasma reactor for processing a semiconductor wafer has a vacuum chamber 10 enclosed by a cylindrical side wall 12, a ceiling 14 and a floor 16. A wafer pedestal 18 supports a semiconductor wafer 20 which is to be processed. A plasma precursor gas is injected into the chamber 10 through a gas injector 22 from a gas supply 24. Plasma source power is coupled into the chamber 10 in any one of several ways. For example, the reactor may be a “diode” configuration, in which case RF power is applied across a ceiling electrode 26 and the wafer pedestal 18. This is accomplished by connecting the pedestal 18 and the ceiling electrode 26 to either one of two RF power sources 28, 30. Alternatively, a cylindrical side coil 32 wound around the chamber side wall 12 is connected to an RF power source 34. Alternatively to the foregoing, or in addition thereto, a top coil 36 is connected to an RF power supply. As is conventional, the wafer pedestal 18 may have its own independently controllable RF power supply 28 so that ion bombardment energy at the wafer surface can be controlled independently of plasma density, determined by the RF power applied to the coil 32 or the coil 36.

A vacuum pump 40 is coupled to the chamber 10 through a passage 42 in the floor 16. The annular space between the periphery of the wafer pedestal 18 and the floor 16 forms a pumping annulus 44 through which the vacuum pump 40 evacuates gas from the chamber 10 to maintain a desired processing pressure in the chamber 10. The pumping annulus 44 is coupled to the interior of the chamber 10 through an annular gap 46 between the periphery of the wafer pedestal 18 and the chamber side wall 14. In order for the pump 40 to perform efficiently, the gap 46 is preferably free of obstructions.

A conventional slit valve opening 50 of the type well-known in the art having a long thin opening in the chamber side wall 14 provides ingress and egress for a semiconductor wafer 52 to be placed upon and withdrawn from the wafer pedestal 18.

The walls 12, 14 confining the plasma within the chamber 10 are subject to attack from plasma ions and charged radicals, typically, for example, by ion bombardment. Such attack can consume the material in the walls 12, 14 or introduce incompatible material from the chamber walls 12, 14 into the plasma process carried out on the wafer 52, thereby contaminating the process. Such incompatible material may be either the material of the chamber wall itself or may be material (e.g., polymer) previously deposited on the chamber walls during plasma processing, which can flake off or be sputtered off. Plasma reaching the chamber walls can cause polymer deposition thereon.

The openings from the interior portion of the chamber 10, including the pedestal-to-side wall gap 46 and the slit valve opening 50, permit the plasma to leak or flow from the chamber 10. Such leakage can reduce plasma density near the openings 46, 50, thereby upsetting the plasma process carried out on the wafer 52. Also, such leakage can permit the plasma to attack surfaces outside of the chamber interior. The flow of plasma into the pumping annulus 44 through the gap 46 permits the plasma to attack the interior surfaces or walls of the pumping annulus 44. Thus, the designer must typically take into account not only the materials forming the chamber ceiling 12 and side wall 14, but in addition must also take into account the materials forming the pumping annulus, including the lower portion 56 of the side wall 14, the floor 16 and the bottom peripheral surface 58 of the wafer pedestal 18, which complicates the design. Such a loss of plasma from the chamber 10 also reduces plasma density or requires more plasma source power to maintain a desired plasma density over the wafer 52.

Magnetic Confinement

In order to prevent plasma from flowing from the chamber 10 into the pumping annulus, a magnetic field perpendicular to the plane of the gap 46 and perpendicular to the direction of gas flow through the gap 46 is provided across the gap 46. This is preferably accomplished by providing an opposing pair of magnetic poles 60, 62 juxtaposed in facing relationship across the gap 46. In the embodiment according to FIG. 2, the magnetic pole 60 is the north pole of a magnet 64 located at the periphery of the wafer pedestal 18 while the magnetic pole 62 is the south pole of a magnet 66 next to the inner surface of the side wall 14. The embodiment of FIG. 2 may be regarded as an open magnetic circuit because the returning magnetic field lines of flux 68 in FIG. 2 radiate outwardly as shown in the drawing.

In order to prevent plasma from flowing from the chamber 10 through the slit valve opening 50, a magnetic field perpendicular to the plane of the slit valve opening 50 and perpendicular to the direction of gas flow through the slit valve opening 50 is provided across the slit valve opening 50. This is preferably accomplished by providing an opposing pair of magnetic poles 70, 72 juxtaposed in facing relationship across the slit valve opening 50. In the embodiment according to FIG. 3, the magnetic pole 70 is the north pole of a magnet 74 extending across the bottom edge of the slit valve opening 50 while the magnetic pole 72 is the south pole of a magnet 76 extending along the top edge of the slit valve opening 50. The embodiment of FIG. 3 may also be regarded as an open magnetic circuit because the returning magnetic field lines of flux 78 in FIG. 3 radiate outwardly as shown in the drawing.

One potential problem with the returning lines of magnetic flux 68 (FIG. 2) and 78 (FIG. 3) is that some returning flux lines extend near the wafer 52 and may therefore distort or perturb plasma processing of the wafer 52. In order to minimize or eliminate such a problem, a closed magnetic circuit (one in which returning magnetic lines of flux do not extend into the chamber) is employed to provide the opposing magnetic pole pairs 60, 62 and 70, 72. For example, in the embodiment of FIGS. 4 and 5, the opposing magnetic poles 60, 62 across the gap 44 are each a pole of a respective horseshoe ring magnet 80, 82 concentric with the wafer pedestal 18. The horseshoe ring magnet 80 has the north pole 60 and a south pole 81 while the horseshoe ring magnet has the south pole 62 and a north pole 83. The poles 60, 81 of the inner horseshoe ring magnet 80 are annuli connected at their inner radii by a magnetic cylindrical core annulus 85. Similarly, the poles 62, 83 of the outer horseshoe ring magnet 82 are annuli connected at their outer radii by a magnetic cylindrical core annulus 86. The magnetic circuit consisting of the inner and outer horseshoe ring magnets 80, 82 is a closed circuit because the lines of magnetic flux between the opposing pole pairs 60, 62 and 81, 83 extend straight between the poles and, generally, do not curve outwardly, at least not to the extent of the outwardly curving returning lines of flux 68, 78 of FIGS. 2 and 3.

In the embodiment of FIGS. 4A, 4B and 6, the opposing magnetic poles 70, 72 across the slit valve opening 50 are each a pole of a respective long horseshoe magnet 90, 92 extending along the length of the slit valve opening 50. The long horseshoe magnet 90 extends along the top boundary of the slit valve opening 50 while the other horseshoe magnet extends along bottom edge of the slit valve opening 50.

The advantage of the closed magnetic circuit embodiment of FIG. 4 is that the magnetic field confining the plasma does not tend to interfere with plasma processing on the wafer surface.

In the embodiment of FIGS. 7 and 8, the lower annuli 81, 83 of the two horseshoe ring magnets 80, 82 are joined together as a single annulus by a magnetic core annulus 96, so that the horseshoe ring magnets 80, 82 constitute a single horseshoe ring magnet 94 having a north pole 60 and a south pole 62. The core annulus 96 extends across the pumping annulus 44 and can be protected by a protective coating 98 such as silicon nitride. In order to allow gas to pass through the pumping annulus 44, the core annulus 96 has plural holes 100 extending therethrough.

One advantage of the invention is that plasma ions are excluded from the pumping annulus 44. This is advantageous because the pumping annulus interior surfaces can be formed of any convenient material without regard to its susceptibility to attack by plasma ions or compatibility of its sputter by-products with the plasma process carried out on the wafer. This also eliminates reduction in plasma density due to loss of plasma ions through the pumping annulus. Another advantage is that gas flow through the pedestal-to-side wall gap 46 is not obstructed even though plasma is confined to the interior chamber 10 over the wafer. Furthermore, by so confining the plasma to a smaller volume (i.e., in the portion of the chamber 10 directly overlying the wafer 52), the plasma density over the wafer 52 is enhanced. A further advantage is that stopping plasma ions from exiting through the slit valve opening 50 eliminates loss of plasma density over portions of the wafer 52 adjacent the slit valve opening 50.

In one example, each of the magnetic pole pair 60, 62 has a strength of 20 Gauss for a distance across the gap 46 of 5 cm, while each of the magnetic pole pair 70, 72 has a strength of 20 Gauss for a width of the slit valve opening 50 of 2 cm.

While the invention has been described with reference to preferred embodiments in which the plasma confining magnets are protected from attack from plasma ions and processing gases by being at least partially encapsulated in the chamber walls or within the wafer pedestal or within a protective layer, in some embodiments (as for example, the embodiment of FIG. 6) the magnets may be protected by being located entirely outside of the chamber walls. Alternatively, if the reactor designer is willing to permit some plasma interaction with the magnets, magnets may be located inside the chamber in direct contact with the plasma, although this would not be preferred.

While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention. 

What is claimed is:
 1. An RF plasma reactor for processing a semiconductor wafer, comprising: a reactor chamber bounded by a chamber wall defining an interior region of said chamber, and a gas inlet; an RF power source and an RF power applicator proximal said chamber and connected to said RF power source; an opening in said chamber communicating with said interior region of said chamber; a magnet apparatus disposed adjacent said opening to resist flow of plasma ions through said opening; the magnet apparatus comprising a first pair of magnetic poles and a second pair of magnetic poles, the first pair of magnetic poles facing the second pair of magnetic poles across said opening.
 2. A reactor as claimed in claim 1 wherein the opening comprises at least one of a) an annular gap formed between a wafer pedestal and said chamber wall, b) a gas inlet, c) a wafer slit valve, d) a gas exhaust port.
 3. A reactor as claimed in claim 1 wherein each magnetic pole of the first pair of magnetic poles faces a magnetic pole of the second pair of magnetic poles of opposite magnetic polarity.
 4. A reactor as claimed in claim 3 wherein the magnetic circuit formed by the magnetic apparatus is a closed magnetic circuit.
 5. A reactor as claimed in claim 3 further comprising a magnetic material connecting said first pair of poles together.
 6. A reactor as claimed in claim 5 wherein said magnetic material and said first pair of magnetic poles comprise a horseshoe magnet.
 7. A reactor as claimed in claim 1 further comprising a magnetic material connecting said first pair of poles together.
 8. A reactor as claimed in claim 7 wherein said magnetic material and said first pair of magnetic poles comprise a horseshoe magnet.
 9. An RF plasma reactor for processing a semiconductor wafer, comprising: a reactor chamber bounded by a chamber wall, and a gas inlet, said chamber further comprising a primary processing portion; an RF power source and an RF power applicator proximal said chamber and connected to said RF power source; an opening communicating between said primary processing portion and a region external of said primary processing portion; a magnet apparatus disposed at said opening to resist flow of plasma ions through said opening; the magnet apparatus comprising a first pair of magnetic poles of opposite magnetic polarity, a first pole of said first pair of magnetic poles facing a second pole of said first pair of magnetic poles across said gap; and a second pair of magnetic poles, a first pole of said second pair of magnetic poles facing a second pole of said first pair of magnetic poles across said gap, comprising a second portion of a closed magnetic circuit.
 10. A reactor as claimed in claim 9 wherein the opening comprises at least one of a) an annular gap formed between a wafer pedestal and said chamber wall, b) a gas inlet, c) a wafer slit valve, d) a gas exhaust port.
 11. A reactor as claimed in claim 10 wherein the magnetic circuit formed by the magnetic apparatus is a closed magnetic circuit.
 12. An RF plasma reactor for processing a semiconductor wafer, comprising: a reactor chamber bounded by a chamber wall, and a gas inlet, said chamber further comprising a primary processing portion; an RF power source and an RF power applicator proximal said chamber and connected to said RF power source; an opening communicating between said primary processing portion and a region external of said primary processing portion; a magnet apparatus disposed at said opening to resist flow of plasma ions through said opening; the magnet apparatus comprising a first pair of magnetic poles of opposite magnetic polarity, a first pole of said first pair of magnetic poles facing a second pole of said first pair of magnetic poles across said gap; and a magnetic material connecting said first pair of poles together.
 13. A reactor as claimed in claim 12 wherein said magnetic material and said first pair of magnetic poles comprise a horseshoe magnet.
 14. A method of operating an RF plasma reactor for processing a semiconductor wafer, the method comprising: providing a reactor chamber bounded by a chamber wall, and a gas inlet, said chamber further comprising a primary processing portion; providing an RF power source and an RF power applicator proximal said chamber and connected to said RF power source; providing an passageway in said chamber communicating with said primary processing portion; protecting said passageway from the entry of plasma ions by disposing a first pair of magnetic poles and a second pair of magnetic poles adjacent said passageway, the first pair of magnetic poles facing the second pair of magnetic poles across said passageway.
 15. The method of claim 14 wherein the step of providing a passageway comprises of providing at least one of a) an annular gap formed between a wafer pedestal and said chamber wall, b) a gas inlet, c) a wafer slit valve, d) a gas exhaust port.
 16. The method of claim 15 wherein the step of protecting said passageway further comprises forming a closed magnetic circuit across said passageway.
 17. A plasma reactor for processing a workpiece, said reactor comprising: a reactor enclosure defining a processing chamber including an encircling side portion enclosing a processing region of said chamber; a base within said chamber for supporting said workpiece near or in said processing region; a pumping annulus proximal a periphery of said encircling side portion; a passageway between said encircling side portion and a location near said base, said passageway extending from said processing region to said pumping annulus; and a pair of magnets in mutual facing relationship on opposite sides of said passageway, wherein one of said pair of magnets is adjacent said base in such a manner as to be outside of said processing region and the other of said pair of magnets is adjacent said encircling side portion in such a manner as to be outside of said processing region; and wherein said magnet adjacent said base is covered by said base form said processing region and said magnet adjacent said encircling portion is covered by said encircling portion from said processing region.
 18. The reactor of claim 17 wherein said pair of magnets comprise a pair of concentric magnet rings coaxial with an axis of symmetry of said chamber.
 19. The reactor of claim 18 wherein said pair of magnets have their poles oriented so as to establish magnetic field lines across said passageway which tend to deflect plasma ion from traveling through the passageway.
 20. The reactor of claim 19 wherein said pair of magnets have a magnetic pole orientation relative to one another when viewed in cross-section as end-to-end with an opposing pair of poles facing one another.
 21. A plasma reactor for processing a workpiece, said rector comprising: a reactor enclosure defining a processing chamber including a side wall enclosing a processing region of said chamber; a base within said chamber for supporting said workpiece near or in said processing region; a pumping annulus external of said reactor enclosure; a passageway between said encircling side portion said base, said passageway extending from said processing region to said pumping annulus; a pair of magnets in mutual facing relationship on opposite sides of said passageway and outside of said processing region; wherein one of said pair of magnets is adjacent said base and the other of said pair of magnets is adjacent said encircling side portion; and wherein said pair of magnets comprise a pair of concentric magnet rings coaxial with an axis of symmetry of said chamber.
 22. The reactor of claim 21 wherein said pair of magnets comprise a pair of concentric magnet rings coaxial with an axis of symmetry of said chamber.
 23. The reactor of claim 21 wherein said pair of magnets have their poles oriented so as to establish magnetic field lines across said passageway which tend to deflect plasma ion from traveling through said passageway.
 24. The reactor of claim 23 wherein said pair of magnets have a magnetic pole orientation relative to one another when viewed in cross-section as end-to-end with an opening pair of poles facing one another.
 25. In an RF plasma chamber for processing a workpiece and provided with an opening: a first pair of opposed magnetic poles of opposite magnetic polarity, placed so that said opening is positioned therebetween, said magnetic poles being of sufficient magnetic strength to inhibit plasma from advancing into said opening from said chamber; and in which the opposite faces of said first pair of opposed magnetic poles are generally parallel to each other, and of transverse dimension greater than that of said opening, to minimize divergence of magnetic lines of force crossing said opening.
 26. The combination of claim 25 in which the chamber further includes a workpiece support, and in which said magnetic poles are limited in magnetic strength so that the magnetic field adjacent the support is negligible.
 27. The combination of claim 25 in which the chamber includes a workpiece support and in which said magnetic poles are spaced from said workpiece support so that the magnetic filed adjacent the support is negligible.
 28. The combination of claim 25 in which the chamber includes a workpiece support and in which the magnetic poles are positioned so as to direct the lines of magnetic force generally away from said workpiece support.
 29. In an RF plasma chamber for processing a workpiece and provided with an opening: a first pair of opposed magnetic poles of opposite magnetic polarity, placed so that said opening is positioned therebetween, said magnetic poles being of sufficient magnetic strength to inhibit plasma from advancing into said opening from said chamber; and the opposite faces of said first pair of opposed magnetic poles are closely spaced to each other and of traversed dimension greater than but comparable to that of said opening, to concentrate the magnetic field of said poles across said opening.
 30. In an RF plasma chamber for processing a workpiece and provided with an opening: a first pair of opposed magnetic poles of opposite magnetic polarity, placed so that said opening is positioned therebetween, said magnetic poles being of sufficient magnetic strength to inhibit plasma from advancing into said opening from said chamber; and a pair of magnets positioned with said opening therebetween, and with the North poles of one of said magnets facing the South pole of the other of said magnets, said North and South poles comprising said first pair of opposed magnetic poles.
 31. The combination of claim 30 which further includes a magnetic return path connecting the two remaining poles of said magnet pair.
 32. The combination of claim 30 in which the chamber further includes a workpiece support, and said magnetic and return path is spaced from said workpiece support.
 33. In an RF plasma chamber for processing a workpiece and provided with an opening: a first pair of opposed magnetic poles of opposite magnetic polarity, placed so that said opening is positioned therebetween, said magnetic poles being of sufficient magnetic strength to inhibit plasma from advancing into said opening from said chamber; and a magnet positioned to one side of said opening with one pole of said magnet facing generally transversely across aid opening, and a magnetic return path member connected to the other pole of said magnet and terminating in a position opposite and facing said one pole, the terminating end of said magnetic return path member together with said one pole functioning as said first pair of opposed magnetic poles.
 34. The combination of claim 33 in which the chamber further includes a workpiece support, and said magnet and return path member is spaced from said workpiece support.
 35. In an RF plasma chamber for processing a workpiece and provided with an opening: a pair of magnetic poles of opposite magnetic polarity, said poles being positioned with said opening therebetween, said poles being of opposed area and strength to concentrate the magnetic flux across said opening while minimizing same elsewhere within the chamber; a first magnetic member having two pole faces, with said pole faces of said first magnet facing each other in opposed manner with said opening therebetween, whereby the pole faces of said magnetic member comprise said pair of magnetic poles; and the diameter of said pole faces approximate that of said opening.
 36. The combination of claim 35 in which said chamber encloses a plasma processing volume, and in which at least one magnet provides said pair of magnetic poles and is positioned outside the plasma processing volume.
 37. The combination of claim 35 which further includes two magnets positioned so that one magnet has a pole face of one polarity facing a pole face of the opposite polarity of the other magnet, whereby said pole face of one polarity of said one magnet and said pole face of opposite polarity of the other magnet comprise said pair of magnetic poles.
 38. The combination of claim 37 in which the diameter of said pole faces approximates that of said opening.
 39. The combination of claim 37 in which the remaining pole faces of said two magnets are connected by a pole piece providing a magnetic return path.
 40. In an RF plasma chamber for processing a workpiece and provided with an opening: a pair of magnetic poles of opposite magnetic polarity, said poles being positioned with said opening therebetween, said poles being of opposed area and strength to concentrate the magnetic flux across said opening while minimizing same elsewhere within the chamber; and said chamber is provided with a slit valve by which workpieces may be introduced and removed from said chamber, and said poles are positioned on either side of said slit valve.
 41. The combination of claim 35 in which said chamber is provided with a pumping annulus, and said poles are positioned so that said pumping annulus is located therebetween. 